A bifunctional tungstate catalyst for chemical fixation of CO2 at atmospheric pressure.

Chemical fixation of carbon dioxide (CO2) into useful and valuable chemicals is a key technology for sustainable lowcarbon society because CO2 is a renewable and environmentally friendly C1 source, which is in contrast to toxic CO and phosgene. However, CO2 is much less reactive than CO and phosgene, and a large energy input (e.g., highly reactive reagents, high pressures of CO2, and stoichiometric amounts of strong acids or bases) is usually required to transform CO2 into various chemicals. Therefore, the low-energy catalytic fixation of CO2 is highly desirable. Catalytic CO2 fixation at atmospheric pressure has been limited to reactive substrates (strained cyclic molecules, unsaturated compounds, etc.), and bifunctional catalysts, which allow a concerted action on both CO2 and substrates, are promising candidates for highly efficient CO2 fixation at atmospheric pressure. [1d] Catalytic C N and C O bond-formation processes with CO2 are important in both industry and academia because they offer economical and environmental advantages such as high atom efficiency and water is the only by-product. Urea derivatives, made from CO2, are important end products or intermediates for pharmaceuticals, agricultural pesticides, antioxidants in gasoline, dyes, and resin precursors. Although various base catalysts such as CsOH, Cs2CO3, and ionic liquids have been used for the synthesis of urea derivatives with CO2, these systems have disadvantages: 1) high CO2 pressures (2.5–8.0 MPa) and reaction temperatures (423–453 K), 2) narrow applicability to substrates, and 3) need of dehydrating agents or additives (see Table S1 in the Supporting Information). Recently, we have developed a series of polyoxometalates (POMs) as catalysts for various functional-group transformations. While acid and oxidation catalysis by POMs have extensively been investigated, there are no successful examples of base catalysis including chemical fixation of CO2. [4,5] The charges and sizes of POMs strongly affect the basicities of the oxygen atoms, and increase with an increase in the charge densities (i.e., the negative charge per size). On the basis of this concept, we focus on the basic property of a monomeric tungstate, [WO4] 2 , having a high charge density. First, the structures of various tungstates were optimized by the density functional theory (DFT) calculations, and the basicities of oxygen atoms in various POMs were compared with the natural bond orbital (NBO) charges (Figure 1a). The NBO charge of an oxygen atom in [WO4] 2 was 0.934, which is